An in-cell touch panel is formed by integrating a touch panel (TP) as an input medium together with a display panel, and plays an important role in the display technical field. Mutual capacitive TPs are very popular due to their advantages of high sensitivities and multi-touch.
An in-cell touch panel is such a device that the touch drive electrode line and the touch sense electrode line of the touch panel are integrated in the display panel. For example, the touch drive electrode line and the touch sense electrode line are integrated in a Liquid Crystal Display (LCD) or an Organic Light Emitting Device (OLED). The touch drive electrode line and the touch sense electrode line may be fabricated on the front substrate and/or rear substrate of the display panel. To simplify the configuration and reduce the thickness of the in-cell touch panel, the gate line, the common electrode line and other functional electrode lines of the display panel may be used as the touch drive electrode, which is driven in a time division manner to realize image display and touch function.
The basic principle of operation of the mutual capacitive touch panel will be briefly described in the following.
The touch drive electrode of the mutual capacitive touch panel determines X coordinate of the touch point location and the touch sense electrode determines its Y coordinate. A touch drive voltage is applied to the touch drive electrode and a constant voltage is applied to the touch sense electrode. When detecting a touch point location, touch drive electrodes along the X direction are scanned row by row. Signals at individual touch sense electrodes are read when scanning each row of the touch drive electrode. A round of scan can traverse all intersections between each row and each column and totally X*Y signals are scanned. With such a method for detecting the touch point location, coordinates of multiple points may be determined, and thereby realizing multi-touch.
A typical and conventional circuit for detecting a touch point location on a touch panel is illustrated in FIG. 1, which comprises: a sensing sub-circuit 101, an amplification sub-circuit 102, an output sub-circuit 103 and a detection sub-circuit 104. The sensing sub-circuit 101 comprises a fixed capacitor C1, a variable capacitor Cf and a Thin Film Transistor (TFT) M1. The gate and source terminals of the TFT M1 are connected to a touch drive electrode line (such as the gate line Gate(n−1) illustrated in FIG. 1, respectively, which is multiplexed as the touch drive electrode line and the gate line in a time division manner) and a reset voltage line (Vint line), the drain terminal of the TFT M1 is connected to a terminal of the variable capacitor Cf, and the other terminal of the variable capacitor Cf is connected to a reference voltage. A terminal of the fixed capacitor C1 is connected to the gate line Gate(n−1), and the other terminal is connected to the drain terminal of the TFT M1. The amplification sub-circuit 102 comprises a TFT Mamp for amplifying signals. The gate terminal of the TFT Mamp is connected to the drain terminal of the TFT M1, the source terminal is connected to the Vint line, and the drain terminal is connected to the source terminal of a TFT M2 in the output sub-circuit 103. The gate terminal of the TFT M2 is connected to another touch drive electrode line (such as the gate line Gate(n) illustrated in FIG. 1), and the drain terminal is connected to the detection sub-circuit 104 via a Read Out line.
The principle of operation of the circuit for detecting the touch point location on the in-cell touch panel as illustrated in FIG. 1 is as follows: when the Gate(n−1) is at a high level, the TFT M1 is turned on, the fixed capacitor C1 and the variable capacitor Cf are charged, and the drain terminal of the TFT M1 (i.e., node Vc illustrated in FIG. 1) will be charged to Vint (that is, Vc=Vint). When the Gate(n−1) is at a low level, voltage at the node Vc is changed to the following value due to a the capacitance coupling effect:Vc=Vint−C1*ΔVp/(C1+Cf)  (1)
In the above Equation (1), ΔVp represents a difference between a high voltage and a low voltage of a linear pulse on the Gate(n−1) line. When a touch occurs, the capacitance of Cf will be changed (generally increased), and thus the voltage at the node Vc is changed. That is, the gate voltage of the amplifying TFT Mamp is changed, the source current of the TFT Mamp is accordingly changed, that is, the source current flowing from the TFT Mamp to the TFT M2 is changed. When Gate(n) is of a high level, the TFT M2 is turned on, and a current flowing from the drain of M2 to the detection sub-circuit 104 via the Read Out Line is changed. The location of the touch point can be accordingly determined by detecting the changed current by the detection sub-circuit 104.
The main disadvantage of the circuit for detecting the touch point location on the in-cell touch panel illustrated in FIG. 1 is as follows: when comparing the value of Vc in Equation (1) computed with the changed Cf caused by the touch with the value of Vc in case of un-touch, the Vc change between the touch and un-touch is not significant and thus the change in the current flowing from the TFT Mamp to the Read Out Line via the TFT M2 is not as significant. As a result, the change of voltage Vout output by the detection sub-circuit 104 is not apparent, so the detection precision of the sub-circuit is low, that is, the precision of touch point location determination is not high. Moreover, since the amplification effect of the amplification sub-circuit 102 in the circuit for detecting the touch point location on the in-cell touch panel illustrated in FIG. 1 is not apparent, the current change amount at Read Out Line is small and the precision of the touch point location is low.